The canister purge system controls the flow and rate of flow of fuel vapors from the collection canister to the intake manifold. One known type of canister purge system comprises a solenoid-operated valve which is under the control of the engine electronic control unit (ECU). A signal from the ECU to the valve solenoid determines the extent to which the valve restricts the flow of vapors from the canister to the manifold. Under conditions that are unfavorable to purging, the valve is fully closed. As conditions become increasingly favorable to purging, the valve is increasingly opened.
A suitably designed and operated pulse-width modulated solenoid-operated valve can exercise a rather precise degree of control over the purging, especially at those times when only small purge flow rates are permissible. On the other hand, compliance with a requirement for such precise low-flow control may limit the valve's capacity for handling much larger purge flow rates. Stated another way, building a higher flow version of the known valve will compromise low flow resolution, de-grading the control resolution at engine idle. Moreover, continued usage of the typical, fairly low, modulation frequency (10-16 hz) for higher flow rate control can introduce pulsations that adversely affect hydrocarbon constituents of engine exhaust.
The present invention is directed to a canister purge system that exhibits accurate control at low flow rates, and yet will handle much larger flow rates in a very acceptable manner. This capability is attained by the combination of a canister purge solenoid valve having an inlet, an outlet, and a valving means that is disposed in a passage between the inlet and outlet and imposes a selected restriction to flow through this passage in accordance with an electrical control signal delivered to the valve solenoid, and a normally-closed, vacuum-actuated valve having an inlet, an outlet, and a valving means that is disposed in a passage between the last-mentioned inlet and outlet and opens the last-mentioned passage to flow only for values of a vacuum signal input to a control port of the normally-closed, vacuum-actuated valve which exceed a certain minimum, first conduit means, including orifice means, for connecting the inlet and outlet of the canister purge solenoid valve to a canister and an engine intake manifold respectively, second conduit means for connecting the inlet and outlet of the normally-closed, vacuum-actuated valve to the canister and engine intake manifold respectively, and third conduit means connecting the control port of the normally-closed, vacuum-actuated valve to a tap that is disposed in that portion of the first conduit means which is between the orifice means and the canister purge solenoid valve.
In a first embodiment that is specifically illustrated in the drawings, the tap is disposed between the orifice means and the inlet of the canister purge solenoid valve, the canister purge solenoid valve and the normally-closed, vacuum-actuated valve are separate assemblies, and all three of the conduit means are external to the two valves.
In a second embodiment that is illustrated in the drawings, the two valves and orifice means are integrated into a unitary assembly.
A third embodiment that is specifically illustrated in the drawings and is like the first embodiment includes a pressure regulator disposed in that portion of the first conduit means between the outlet of the canister purge solenoid valve and the intake manifold. The pressure regulator compensates for changes in intake manifold vacuum such that over the effective range of the regulator the purge flow set by the solenoid-actuated valve through the first conduit means is rendered substantially unaffected by changes in intake manifold vacuum.
A fourth embodiment that is specifically illustrated in the drawings and is like the third embodiment includes the two valves and the pressure regulator integrated into the unitary assembly. The pressure regulator performs the same function in this fourth embodiment as does the pressure regulator of the third embodiment.
Further details and advantages of the invention will be seen in the ensuing description and claims, which should be considered in conjunction with the accompanying drawings. A presently preferred embodiment of the invention discloses the best mode contemplated for carrying out the invention.